How to determine the right gear ratio for your car
Rear end gears are EXTREMELY important for how a car performs, whether it be for drag racing, road racing, 1/2 or 1 mile racing, and even the impact on daily driving, for those dual purpose vehicles. In fact, they are every bit as important as the engine, modifications and even the driver. If you don’t have the right gear ratio, even with a super powerful engine, your car can end-up being a complete non-performer.
When deciding on a set of rear end gears, you have to take into consideration and know what RPM you’ll be at for any given speed you’ll be traveling. If you want your car to be quick off the line and launch hard with a super quick 0 – 60 MPH time, you’ll need low or short gears. If you want your car to be a top speed monster, you’ll need high or tall gears. The numbers are deceiving because a high gear ratio has a low number, and a lower gear ratio has a higher number. They are ratios, so the numbers can fool you. A really low set of gears would be 513’s . It’s a low gear ratio but it has a high number. A really tall set of gears would be something like 311’s.
Here are a few examples: First, rear end gears are no different than the sprockets you have on any given 10 speed bicycle. To do wheelies and be able to climb steep hills, you need low gears. You can’t do a wheelie in anything higher than 4th gear on a typical 10 speed bike because the gear ratio is just too tall. In retrospect though, you aren’t going to go fast down the road if you are in 1st, 2nd or 3rd gear. More of an example would be putting the bike in 5th gear and trying to do a wheelie, not going to happen. Further, put the bike in 1st or 2nd gear and try to ride 30 MPH, again, not going to happen. You won’t be able to because your legs can’t spin the pedals fast enough. You’ll run out of RPM and will need to shift to a higher gear. Rear end gears on cars are no different, and having the wrong gear ratio is exactly like trying to do a wheelie in 5th gear, or trying to go down the road at 30 MPH in 1st. Both attempts will fail because the gear ratios are completely wrong for what you are trying to accomplish.
Drag racing cars need low gearing so they can launch as hard a possible, and get to the 1/8th or 1/4 miles finish as soon as possible. distance cars, like 1/2 or 1 miles runs, or even something like a Bonneville race car needs tall gears to be able to reach the high speeds over longer distance they need. Another way to look at it; the shorter the gear the faster you get to the finish line for a short distance race, and the taller the gear the higher stop speed you go in a longer distance, though it takes you longer to get to that speed. When customers want to make their cars perform more, and don’t know any better, the only thing they look at and consider is the engine. The engine is literally only part of the battle. The gearing is the other half.
More examples: A car at the drag strip with 4.11 gears and a 300 HP engine will launch harder and get off the line faster and will be quicker than a car that has 500 HP and gears that are way too tall. 80% of the race is the first 80 feet! If you can’t launch and get to your speed quickly, you won’t run a good elapse time for that short race. It also means you’ll get your doors blown off in a street race from stoplight to stoplight. You can brag all you want that your engine has 500 HP when your buddy only has 300, but if he has the right gearing, his little 300 HP car is going to send you home crying to mommy when you lose a race from a dead stop to 75 MPH or so, which is where 99% of street races happen.
So, how do I know which gear ration I need?
The best way to figure out what gear ratio is best for your car is to first make a solid decision on how and where you’ll be driving the car. If you want to drive on the streets, do easy burnouts, be quick on the street, and look great at the weekly drags, then you’ll want a set of gears that matches that style of driving, which in this case would be low gears. The thing you need to do is determine what MPH you think your car is capable of hitting, or what your max RPM is for the distance and transmission gear, based on cams, valve springs and where your engine and tune maxes out on the power curve.
If your engine power peaks at 6,400 RPM, then there’s no reason to rev beyond that point because you’ll only be slower. Too many like to brag about how high their engine can rev, which just shows how much they don’t know about cars, engines, and performance. Sure, many engines can rev way past their peak horsepower, but it’s counter productive to run it past that point because you won’t be accelerating at the same speeds. You always go by where it peaks out on power… nothing higher in RPM than that or you’re wasting time…slower ET’s!
So how do I calculate gear ratio needed?
The next thing you need to do is measure the diameter of the rear tires you’re running. Let’s use 28″ for this baseline. To determine how fast you can go at the 6,400 RPM limit with 28″ tall tires, some simple math is used to calculate it with any gear ratio. The math is: RPM x tire diameter, so 6400 x 28″ = 179200. Now take the gear ratio you want to check against and times it by 336. I’m going to use 4.11’s as a baseline, so 4.11 x 336 = 1380.96. Now divide the 179200 into the 1380.96 and we get 129.79. That means our top speed at 6,400 RPM with a 28″ tall rear tire will be right at 130 MPH.
Torque converter slip is also something very important to keep in mind too! Even though 6,400 RPM is much higher than what hot street cars run for stall converters, most are usually around 3,000 RPM or so, they still have some slippage of 200 or 300 RPM or at high RPM, so you need to be aware of this when calculating any of these equations. This is especially true when trying to determine what RPM you’ll be at when traveling down the freeway at 60 MPH because at that low of an RPM, your 3,000 RPM stall converter is slipping several hundred RPM which will throw you a curve ball when trying to calculate these numbers. In other words, the math might say you’ll be at say, 2,700 RPM at 60 MPH, but if you have a slippy converter, such as a 3,000 – 3,500 stall, you can easily have 500 or more RPM of slippage which will add to that cruise RPM you calculated. So the reality is, you might THINK you’ll be at 2,700 RPM but with the converter slippage you could really be at something more like 3,200 RPM or more!
So looking at that 129.79 (130) MPH number we calculated, that’s within the quarter mile time of a typical low to mid 10 second car. If you think your car can’t run 10’s then you can adjust the gear ratio up or down to suit what speed or RPM you want to go through the finish line at. Let me give you an example; swapping those 4.11 gears out for a set of 4.56’s and using the same numbers, you’ll be going through the finish line at 116.96 (117) MPH at 6,400 RPM. Not as “fast” mile per hour wise, but you’ll launch harder and will get there quicker.
Let’s take this same set-up and see what it does with a typical set of stock street gears of 3.25’s. 6,400 RPM x 28″ tall tires = 179200. 3.25 gears x 336 = 1092. Divide 179200 by 1092 and you get 164.10 MPH. In other words, with a typical street car that has 400 – 450 HP you’ll NEVER be able to get to 164 MPH in a quarter mile, let alone 1 full mile because most cars will float and lose control at that speed without suspension modifications, ground effects, etc, so that gear ratio is completely useless for drag or street racing, although it’s a great ratio for running down the highway at a decent RPM for long distances.
So let’s take a look at running down the freeway using this same car set-up and numbers. If you aren’t into drag or street racing and you live in an area where you mostly do freeway driving for extended periods of time, you won’t want (or even care about) a low gear ratio for jack rabbit starts. You’ll instead want something that’ll get you better gas mileage and cruise down the highway at a decent RPM. So let’s look at a couple of equations for determining what speed and RPM you’ll be at for a given gear ratio and tire diameter. I’m going to use 70 MPH as our cruise speed with the same 28″ tire diameter we used before. This is a slightly different equation so pay attention to the numbers. 70 MPH x the axle ratio you want to see about x 336. So 70 x let’s try a gear set of 3.50’s, x 336 = 82320. Now divide that number into the tire diameter which will give us the RPM at 70 MPH. 82320 28″ = 2,940 so at 70 MPH with 3.50 gears and 28″ tall tires you’ll be at 2,940 RPM.
Again, you must take into consideration torque converter slip! If you have a slippy converter of say, 3,000 RPM, then you are definitely going to see 300 or so more RPM than what the math just gave you unless you have a lock-up converter OR you are running a manual transmission. another very important thing to consider is what type of tranny you are running. In other words; does it have an overdrive? All of this math is for a 1:1 high gear ratio which is what all TH-350, TH-400, C-4, C-6, FMX automatics have for high gear, as well as pretty much all 3 and 4 speed manual transmissions. If you have an overdrive tranny then you can calculate the number by dividing the overdrive percentage into the final number. If you have a .80 overdrive (20% overdrive), then remove 20% from that RPM number and you’ll be in the ball park for what your 70 MPH cruise RPM will be.
So let’s look at that same scenario with 3.00 gears just so we “get’ the math better. 70 MPH x 3.00 gears x 336 = 70560 Divide that by the 28″ tall tire diameter that I chose which = 2,520. So with 3.00 gears that same car at 70 MPH dropped from 2,940 RPM to 2,520. That means there is a 420 RPM difference between 3.00 gears and 3.50 gears on that car.
More ways to skin this cat!
Let’s say you already have an idea of how fast you want your car to be and at what RPM the engine peaks out at and you want to know what gear ratio to go with so the engine’s RPM maxes out at the top speed you want to go. This is another simple equation. So let’s first determine the baseline numbers. Let’s say it’s a 66 Mustang with a hot little 331 stroker in it that dyno’d at 410 HP at 5,670 RPM, so we’ll call it 5,700 RPM max. The car is going to be used on the street and maybe some weekend drag racing just for some fun. It isn’t realistic to think a 331 with 410 HP will propel a car like that to 150, or even 120 MPH on a drag strip, so let’s say 110 MPH is a reasonable top speed for that car for either at the track or for running down the open back roads. A Mustang usually has a pretty short tire diameter of like 25 inches, so we’ll use 25″ as our baseline. So we don’t want to go through the finish line at any higher RPM than 5,700 RPM and we want the top speed to be 110 so the math is: 5,700 x the tire diameter (25) which = 142500. Now we take the 110 MPH and x it by 336 which = 36960. Now take 142500 and divide it by 36960 to give us the gear ratio of 3.86. There are some 3.80 gears out there but for most gear manufacturers only offer 3.73’s and 4.10’s. Occasionally you’ll find 3.89’s or 3.91’s, but just to keep it simple, let’s say your choices are either 3.73’s or 4.10’s. This is an easy one to figure out. If you went with the 3.73’s, at 110 MPH you’d be spinning 5,514 RPM, which you might think is a little low for what you want, BUT if you’re running an automatic transmission with a stall converter of say 2,600 RPM it will have a little bit of slippage, even at that RPM, so figure on about another 200 – 300 RPM which would make that RPM more like 5,700 – 5,800 which is pretty much right on the money for where you want to be at.
Just for giggles, if you chose to run the 4.10 gears, at 110 MPH you’d be spinning 6,061 RPM. Now add the torque converter slippage of a couple of hundred RPM and you’re up into the 6,300 RPM area which is just too high for an engine that peaks in power at 5,700 RPM. In almost all cases it’s better to go less in RPM than too much. It’s also easier on the engine and components.
Here are the math equations:
To determine what RPM you’ll be at for any given gear ratio and tire diameter:
MPH x Axle Ratio x 336 — Divide that number by the Tire Diameter
To determine what speed you’ll be going with a given tire diameter and gear ratio:
RPM x Tire Diameter — Divide that number by the Axle Ratio x 336
To determine what axle ratio you’ll need for a given speed and tire diameter:
RPM x Tire Diameter — divide that number by the MPH x 336
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